Fix typo spotted by Richie Hindle

Doc/lib/libpdb.tex

@@ -262,7 +262,7 @@ bottom-most frame.  This lets you jump back and execute code
 again, or jump forward to skip code that you don't want to run.
 
 It should be noted that not all jumps are allowed -- for instance it
-it not possible to jump into the middle of a for loop or out of a
+is not possible to jump into the middle of a for loop or out of a
 finally clause.
 
 \item[l(ist) \optional{\var{first\optional{, last}}}]